1. Field of the Invention
This invention relates to the composition and structure of thin film multi-layer magnetoresistive sensors. More specifically, the invention relates to the application and composition of free layers having iron contents greater than 50 atomic %, to magnetic tunnel junction (MTJ) sensors.
2. Description of the Related Art
MTJ sensors typically employ a multi-layered structure which includes a tunnel barrier layer positioned between two groups of ferromagnetic layers. The entire multi-layer structure is often referred to as a “stack”. The tunnel barrier is a very thin dielectric layer, while the two groups of ferromagnetic layers are typically formed of a plurality of electrically conductive ferromagnetic materials and layers. On one side of the tunnel barrier, the magnetization direction of the ferromagnetic layers is “pinned” and provides a reference direction for the MTJ head. However, the magnetization direction of the ferromagnetic layers (the “free layers”) formed on the other side of the tunnel barrier rotates freely in response to an external magnetic field from the magnetic medium proximate to the ABS. As the magnetization of the freely rotating ferromagnetic layer rotates in response to the external magnetic field from the magnetic medium, the resistance of the tunnel barrier changes, which can be measured as a change in resistance of the MTJ sensor.
The elemental composition of the free layers in the MTJ sensor have a major impact on the magnetic properties of the sensor, and therefore impact the overall performance. For good MTJ sensor performance, the free layer should have low coercivity (Hce), low magnetostriction, Lambda (λ), high tunneling magneto-resistance (TMR) ratio, and low resistance times area product (RA). An optimum balance of these parameters has been reported in the prior art, disclosing amorphous Fe—Co—B alloy free layers having Fe concentrations less than 15-20 atomic %, and Fe:Co ratios of 1:9. Although free layer Fe concentrations as high as about 50 atomic % have been reported in the prior art, the performance of these layers is not balanced, in that they trade good performance in a single parameter (such as TMR ratio) at the expense of other parameters.
A summary of relevant prior art is provided below.
United States Patent Application Publication 2004/0246632 discloses a magnetoresistive head in which a pinned layer comprises two films, i.e., a ferromagnetic film A and a ferromagnetic B anti-ferromagnetically coupled to each other and a anti-ferromagnetic coupling film for separating the two ferromagnetic films A and B, where the coercivity of the ferromagnetic film alone is 200 (Oe) or more and the coercivity of the ferromagnetic film alone is 20 (Oe) or less. The compositions for the ferromagnetic film A and the ferromagnetic film B, when expressed by Co(100-Y)FeY (at %) are: ferromagnetic film A: 80>Y>40, and ferromagnetic film B: 20>Y>0, where the material for the film in contact with the ferromagnetic film A is Ru, Ta, Ni—Fe—Cr alloy, Cu or Ni—Fe alloy.
United States Patent Application Publication 2005/0110004 discloses a magnetic tunnel element that can be used, for example, as part of a read head or a magnetic memory cell, including a first layer formed from an amorphous material, an amorphous tunnel barrier layer, and an interface layer between the first layer and the tunnel barrier layer. The interface layer is formed from a material that is crystalline when the material is in isolation from both the first layer and the tunnel barrier layer. Alternatively, the thickness of the interface layer is selected so that the interface layer is not crystalline. The first layer is formed from at least one material selected from the group consisting of amorphous ferromagnetic material, amorphous ferrimagnetic materials, and amorphous non-magnetic materials. The interface layer is formed from a material selected from the group consisting of a ferromagnetic material and a ferrimagnetic material.
United States Patent Application Publication 2005/0168888 discloses a magnetoresistive sensor comprising a pinned layer having a magnetization direction fixed with respect to an external magnetic field, a free layer, having a magnetization direction variable in accordance with the external magnetic field, and a spacer layer mainly containing copper, sandwiched between the pinned layer and the free layer. A sense current flows through the pinned layer, the spacer layer, and the free layer substantially in a direction in which the layers are stacked. The free layer comprises at least one intermediate stack composed of a non-magnetic layer mainly containing copper, and a first cobalt iron layer made of a cobalt iron alloy and disposed on boundaries on both sides of the non-magnetic layer, nickel iron alloy layers disposed on boundaries on both sides of the intermediate stack, and a second cobalt iron layer made of a cobalt iron alloy and formed in contact with the spacer layer on a boundary, opposing the spacer layer, of a stack composed of the intermediate stack and the nickel iron alloy layer.
United States Patent Application Publication 2005/0185454 discloses methods of manufacturing MTJ memory cells and structures thereof. A diffusion barrier is disposed between an anti-ferromagnetic layer and a pinned layer of an MTJ memory cell to improve thermal stability of the MTJ memory cell. The diffusion barrier may comprise an amorphous material or a Ni—Fe alloy. An amorphous material may be disposed adjacent a bottom surface of a tunnel junction, within a free layer, or both. An MTJ memory cell with improved thermal stability and decreased Neel coupling is achieved.